Broadcast transmission systems and the like that require high reliability establish redundancy by providing a plurality of active systems and backup systems. In such systems, switchable apparatuses combine active devices and backup devices with switchers in series connection. If there is a causal relationship with respect to an abnormality in a device in an earlier-stage apparatus and an abnormality in a device in a later-stage apparatus, an abnormality occurring in the earlier-stage apparatus is detected not only by the earlier-stage apparatus but also by the later-stage apparatus. If an abnormality occurs in the earlier-stage apparatus, the abnormality is also detected in the later-stage apparatus until the switching of devices in the earlier-stage apparatus is completed.
If an abnormality is detected in the later-stage apparatus due to the abnormality of an earlier-stage apparatus, switching the device in the later-stage apparatus can not resolve the abnormality in the later-stage apparatus, it is necessary to wait for the switching of the device in the earlier-stage apparatus. That is, if an abnormality is detected in a later-stage apparatus, after confirming that the abnormality occurring in an earlier-stage apparatus is not the cause, it is necessary to switch the device in the later-stage apparatus. There are methods for avoiding unnecessary switching of the device in the later-stage apparatus. As an example of the methods, if the abnormality is detected in the later-stage apparatus, after a waiting time has elapsed, the devices are switched based on whether or not an abnormality exists in the later-stage apparatus. The waiting time is a time required for switching the devices in the earlier-stage apparatus
In the above-described method, however, if the abnormality occurs in the later-stage apparatus, because the switching of the device has not been done until the waiting time has elapsed, a state might be continuously occurred in which the abnormality has occurred. Additionally, in the system in which a number of devices are serially connected, if the abnormality occurs in a later-stage apparatus, it means that a state of the abnormality continues for a long time so that the operation of the system might be hindered.